It is known that in biological treatment of water or wastewater, the water is passed through some type of reactor or several reactors (a vessel or another space) wherein micro-organisms are utilized for converting pollutants in the water to harmless end products such as carbon dioxide and water. The treatment can be performed under supply of air (aerobically) or without supply of air (anaerobically) or without supply of air but with presence of significant amounts of nitrate (anoxically). In order to increase the efficiency of the treatment process, it is common to aim at a high content of active micro-organisms in the process by preventing such organisms to escape together with the treated water, either by allowing the micro-organisms to grow suspended in the reactor and separating them from the water in a separation stage after the reactor and returning the micro-organisms to the reactor (e.g. the activated sludge process), or by introducing some kind of support material into the process on the surfaces of which the micro-organisms can grow as a biofilm and thus be retained in the process (the biofilm process). There are also mixtures of these two process types, referred to as hybrid processes, wherein the support material is introduced into the activated sludge process so that suspended micro-organisms as well as biofilm growing microorganisms can be utilized in the process.
The biofilm process has several advantages compared to the activated sludge process. For example, higher loads can be applied and the biofilm processes are substantially less sensitive to variations and disturbances. Most conventional biofilm processes are based on packing of carrier material in the treatment reactor, said material comprising fill bodies or blocks which are maintained fixed and immovable in the process. These embodiments of the process involve the risk of clogging of the biofilm bed by biomass or another particulate material and formation of dead zones in the process, wherein the contact between the water and the active micro-organisms is unsatisfactory.
In another type of biofilm process, which has become very successful during the last 20 years, a carrier material which is kept in suspension and in movement is utilized in process referred to as the MBBR process, i.e. “Moving Bed Biofilm Reactor”. The carrier material with micro-organisms growing thereon is maintained in the process by passing outgoing water through a strainer (sieve or grid) having an aperture diameter or slot width which is so small that the carrier material cannot pass therethrough. The advantage of this kind of process is i.a. that the risk of clogging of the bed and formation of dead zones is eliminated.
The use of a carrier material which is kept in suspension and movement in the process was originally reported for different hybrid process applications, i.e. suspended carriers were supplied to activated sludge processes in order to improve the function thereof. Carriers which have been used for this purpose include pieces of foamed rubber (EP 0 142 123), different types of cylindrical fill bodies (Bundesministerium für Forschung and Technologie, “Einsatz von Schwebekörper zur Erhöung der.” by Dr. D. Dengler, H. Lang, A. Baum, Forschungsbericht 02 WA 8538, January 1988, pages 12 and 13), carriers including hemispherical bodies having inner walls (DE 30 17 439), “hedgehog-like” carriers, perforated spheres, and crossed plates (EP 0 058 974).
WO 96/10542 discloses another embodiment that is related to the foamed rubber pieces of EP 0 142 123. Thin flakes of foamed plastic are used that provide protected surface area in the randomly formed pores of the carriers.
Since the carriers in the MBBR process are exposed to repeated collisions with each other and other surfaces in the reactor, the surfaces that are exposed to other carriers or other surfaces in the reactor are kept clean from biofilm growth. The efficiency of the process therefore is highly dependent on the area that is protected against collisions, for example in inner passages or compartments in the carriers.
Another embodiment of MBBR carriers is disclosed in FR 0850478, which concerns a method for biological treatment of water in which is used carriers, which are plate-formed bodies with protrusions jutting out from the plates, said protrusions being separate from each other and where the protrusions and the body itself both provide protected surface area. This embodiment is claimed to give mass transfer advantages when the carriers are stacking close together as the water can still move between the body and protrusions.
EP 1 431 252 discloses a carrier element that is probably suitable for an MBBR process. The carrier element is said to have excellent properties when it comes to the ability to retain air bubbles, meaning that the aeration process will be more efficient.
WO 00/15565 discloses a “radial flow bioreactor”, usable e.g. for treatment of wastewater. In FIGS. 13-15 thereof, different embodiments of carrier elements exhibiting growth areas allowing for a biofilm having a desired thickness are shown. It is, however, highly unlikely that these carrier elements can be used as free-flowing carrier elements. In any event, they are extremely inefficient, since the protected area amounts to less than 50% of the area of the carrier element's surface area.
One problem with all known carriers for water treatment is that they allow for practically unlimited growth of the thickness of the bioactive microbe layer. This might lead to problems with restricted flow and/or anaerobic zones in the biofilm, even in an environment designed to be aerobic.